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Crystal twinning occurs when two separate crystals share some of the same Crystal Lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A Twin Boundary or composition surface separates the two crystals. Crystallographer s classify twinned crystals by a number of Twin Laws . These twin laws are specific to the Crystal System . The type of twinning can be a diagnostic tool in mineral identification. Simple twinned crystals may be ''contact twins'' or ''penetration twins''. Contact twins share a single composition surface often appearing as mirror images across the boundary. Quartz , Gypsum , and Spinel often exhibit contact twinning. In penetration twins the individual crystals have the appearance of ''passing through'' each other in a symmetrical manner. Orthoclase , Staurolite , Pyrite , and Fluorite often show penetration twinning. If several twin crystal parts are aligned by the same twin law they are referred to as multiple or repeated twins. If these multiple twins are aligned in parallel they are called ''polysynthetic twins''. When the multiple twins are not parallel they are ''cyclic twins''. Albite , Calcite , and Pyrite often show polysynthetic twinning. Closely spaced polysynthetic twinning is often observed as Striation s or fine parallel lines on the crystal face. Rutile , Aragonite , Cerussite , and Chrysoberyl often exhibit cyclic twinning, typically in a radiating pattern. There are three modes of formation of twinned crystals. ''Growth twins'' are the result of an interruption or change in the lattice during formation or growth due to a possible deformation from a larger substituting ion. ''Transformation twins'' are the result of a change in crystal system during cooling as one ''form'' becomes unstable and the crystal structure must re-organize or ''transform'' into another more stable form. ''Deformation'' or ''gliding twins'' are the result of stress on the crystal after the crystal has formed. Deformation twinning is a common result of Regional Metamorphism . Of the three common crystal structures: BCC, FCC, and HCP, the HCP (hexagonal close-packed) structure is the most likely to twin. Crystals that grow adjacent to each other may be aligned to resemble twinning. This ''parallel growth'' simply reduces system energy and is not twinning. REFERENCE
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